Several patents have issued since the commercial success of electric discharge machining (hereinafter "EDM") relating to the devices which can hold EDM tooling so that it may be repeatably placed in operable position in the EDM machine and removed therefrom. Typically such devices, which are herein termed "tool holders", may be repetitively positioned with great accuracy in and thereafter removed from the EDM machine many times during the course of a production run.
In the present state of the art, EDM tooling, also sometimes referred to as EDM electrodes, is formed from stock which is mounted in a tool holder, the tooling is then created, and thereafter remains with the holder throughout the useful life of the tooling. The tooling is created in the tool holder to avoid the problem of accurately positioning the tooling in the proper X, Y and Z orientations in the tool holder. The tool holder has certain locating devices that enables the holder (and in turn the tooling) to be repeatably positioned very accurately in the EDM machine. Thus, for each EDM tool, there is a tool holder. Tool holders of the prior art useable in EDM machines are shown in the following representative U.S. Pat. Nos. 4,621,821; 4,815,780; 4,855,558; 5,065,991; 5,415,382.
Presently, EDM tool holders are machined of metal. Such holders typically sell for $40.00 or more. As there must be one such holder for each EDM tool, users of such tooling tie up substantial monies just in the cost of the holders. Accordingly, inventory costs merely for tool holders become a significant item in a company's tooling inventory.
In addition to involving significant inventory costs, the prior art tool holders are made up of a collection of individual parts. For example, in the case of U.S. Pat. No. 4,815,780, there is a centering disk 13 which is held to the surface of a mounting plate 6 of the tool holder 2 by four screws 51. The manufacture and assembly of these separate pieces not only is expensive but should this plate shift as a result of the tool holder being bumped, a $60,000 die being machined may be ruined because the tool holder was not positioned accurately in the EDM machine when it made its cut in the die. U.S. Pat. No. 5,065,991 discloses a generally similar centering disk and the same shortcomings are present in such arrangement.
In U.S. Pat. No. 4,855,558, tool holder 2 is made of metal and has four machined slots in the upper surface 30 and the flanks are cut to provide slightly bowed recesses such as at 37b as shown in FIG. 4 so that when the rail 68 of the chuck 1 is drawn into the slot, the lip 38d deflects downwardly along the Z-axis. Forming the downwardly bowed recesses involves difficult machining, raising the cost of the tool holders.